LTC4100 datasheet(37/48 Pages) LINER

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LTC1760

1760fa

As is often the case, the wall adapter will usually have at

least a +10% current limit margin and many times one

can simply set the adapter current limit value to the actual

adapter rating (see Figure 9 & Table 1).

Table 1. Common RCL Resistor Values

Adapter

Rating A

RCL Value*

(Ω) 1%

RCL Power

Dissipation (W)

RCL Power

Rating (W)

1.5

0.06

0.135

0.25

1.8

0.05

0.162

0.25

0.045

0.18

0.25

2.3

0.039

0.206

0.25

2.5

0.036

0.225

0.5

2.7

0.033

0.241

0.5

0.030

0.21

0.5

*Values shown above are rounded to nearest standard value.

Table 1 RCL values take into account LTC1760 C-grade 5% tolerance for VCL1.

Extending System to More than 2 Batteries

The LTC1760 can be extended to manage systems with

more than 3 sources of power. Contact Linear Technology

Applications Engineering for more information.

Charge Termination Issues

Batteries with constant-current charging and voltage-

based charger termination might experience problems

with reductions of charger current caused by adapter

limiting. It is recommended that input limiting feature be

defeated in such cases. Consult the battery manufacturer

for information on how your battery terminates charging.

Setting Charger Output Current Limit

The LTC1760 current DAC and the PWM analog circuitry

must coordinate the setting of the charger current. Failure

to do so will result in incorrect charge currents.

Table 2. Recommended Resistor Values

IMAX (A)

RSENSE (Ω) 1%

RSENSE (W)

RILIMIT (Ω) 1%

0.100

0.25

0.05

0.25

10k

0.025

0.5

33k

0.025

0.5

Open or short to VCC2

Warning

DO NOT CHANGE THE VALUE OF RILIMIT DURING OPERA-

TION. The value must remain fixed and track the RSENSE

APPLICATIONS INFORMATION

value at all times. Changing the current setting can result

in currents that greatly exceed the requested value and

potentially damage the battery or overload the wall adapter

if no input current limiting is provided.

Setting Charger Output Voltage Limit

The value of an external resistor connected from the VLIMIT

pin to GND determines one of five voltage limits that are

applied to the charger output value. See Table 3. These

limits provide a measure of safety with a hardware restric-

tion on charging voltage, which cannot be overridden by

software. This voltage sets the limit that will be applied

to the battery as reported by battery. Since the battery

internal voltage monitor point is the actual cell voltage,

you may see higher voltages, up to 512mV higher, at the

external charger terminals due to the voltage servo loop

action. See Operations “Section 3.7” for more information

on the voltage servo system.

Table 3. Recommended Resistor Values for RVLIMIT

VMAX

RVLIMIT (Ω) 1%

Up to 8.4V

0 (Short to ground)

Up to 12.6V

10k

Up to 16.8V

33k

Up to 21.0V

100k

Up to 32.7V (No Limit)

Open or short to VCC2

Inductor Selection

Higher operating frequencies allow the use of smaller

inductor and capacitor values. A higher frequency gener-

ally results in lower efficiency because of MOSFET gate

charge losses. In addition, the effect of inductor value

on ripple current and low current operation must also be

considered. The inductor ripple current ΔIL decreases with

higher frequency and increases with higher VIN.

ΔI

L =

() L()

OUT 1−

OUT

⎛

⎝⎜

⎞

⎠⎟

Accepting larger values of ΔIL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is ΔIL = 0.4(IMAX). In no case should

ΔIL exceed 0.6(IMAX) due to limits imposed by IREV and

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